There are two main techniques for mobility support in IP, as follows:                Bidirectional tunnelling: A mobile host connects to a stationary anchor point with a bidirectional tunnel. The host communicates through the tunnel via a stable IP address from the anchor point's link. The IP address that terminates the tunnel on the mobile-host side at any given point in time is called the mobile host's on-link IP address.        Route optimization: The mobile host communicates via a direct path to a correspondent host. Packets are routed via the mobile host's on-link IP address. IP address substitution at the mobile host's and correspondent host's IP layers ensures that higher protocol layers see the mobile host's stable IP address instead of the variable on-link IP address.        
Both bidirectional tunnelling and route optimization require extra mobility functionality on mobile hosts. This increases the complexity of host implementations and makes mobility support for legacy hosts difficult. Proxy-based IP mobility protocols are being developed to mitigate this. Such protocols do not require mobility functionality to be at the mobile host, but instead at an access router used by the mobile host. The mobile host's access router therefore becomes a proxy of the mobile host, and one of its IP addresses is used as the mobile host's on-link IP address. The access router therefore handles the mobile host's mobility signalling, and ideally the mobile host should not be aware that it is using a proxy-based IP mobility protocol. Proxy Mobile IPv6 (Sri Gundavelli et al.: Proxy Mobile IPv6, draft-ietf-netlmm-proxymip6-18) is currently the main proxy-based IP mobility protocol.
The base specification of Proxy Mobile IPv6 uses bidirectional tunnelling. However, bidirectional tunnelling increases bandwidth utilization and packet propagation delays, due to a sub-optimal packet route via an anchor point. Efforts are underway to extend Proxy Mobile IPv6 to use route optimization (see, for example, Behcet Sarikaya et al.: PMIPv6 Route Optimization Protocol, draft-qin-netlmm-pmipro-00, and Julien Abeille, Marco Liebsch: Route Optimization for Proxy Mobile IPv6, draft-abeille-netlmm-proxymip6ro-00).
Route optimization requires a mobile host to prove to a correspondent host that it is the legitimate user of its stable IP address. This so-called “IP address ownership proof” in general must operate without a pre-existing security or trust relationship between the mobile host and the correspondent host.
One of the main protocols for route optimization in Mobile IPv6, Enhanced Route Optimization (see Jari Arkko, Christian Vogt, Wassim Haddad: Enhanced Route Optimization for Mobile IPv6, RFC 4866), enables a mobile host to prove ownership of its stable IP address by means of generating the stable IP address cryptographically. Specifically, the stable IP address is a function of the public component of the mobile host's public/private key pair, and the mobile host proves ownership of the stable IP address by presenting evidence that it knows the respective private component.
Unfortunately, using the methodology of Enhanced Route Optimization in a Proxy Mobile IPv6 scenario would require the mobile host's access router to learn the mobile host's private key, and it would require the transfer of the mobile host's private key across access routers as the mobile host moves. This would put the mobile host's public key at an increased risk of compromise and is hence unacceptable from a security perspective.
Some attempts have been made to address this problem. For example, Sarikaya et al.: PMIPv6 Route Optimization Protocol, draft-qin-netlmm-pmipro-00, specifies a proxy-based route optimization solution based on Enhanced Route Optimization. It directly moves the mobile host's mobility functionality to the access router, and requires a mobile host's access router to learn the mobile host's private key. It also requires the transfer of the mobile host's private key between access routers as the mobile host moves. Neither of these requirements are desirable from a security point of view.
Julien Abeille, Marco Liebsch: Route Optimization for Proxy Mobile IPv6, draft-abeille-netlmm-proxymip6ro-00, provides support for route optimization only if both the mobile host and the correspondent host are located in a Proxy Mobile IPv6 domain. The advantage of this is that a pre-existing security and trust relationship can be assumed to exist between the proxies of the mobile host and the correspondent host. This relationship is utilized for IP address ownership proofs. The disadvantage of requiring a correspondent host to be in a Proxy Mobile IPv6 domain is that it limits the set of correspondent hosts for which communications can be route-optimized. Correspondent hosts with support for RFC4866 that are not in a Proxy Mobile IPv6 domain are not supported.
Sangjin Jeong, Ryuji Wakikawa: Route Optimization Support for Proxy Mobile IPv6 (PMIPv6), draft-jeong-netlmm-ro-support-for-pmip6-00 considers route optimization with correspondent hosts within a Proxy Mobile IPv6 domain, and correspondent hosts outside a Proxy Mobile IPv6v domain. For the former case, security and trust relationships are assumed to exist between the proxies of the mobile host and the correspondent host. This has the same disadvantages as in Julien Abeille, Marco Liebsch: Route Optimization for Proxy Mobile IPv6, draft-abeille-netlmm-proxymip6ro-00. For the latter case, route optimization is achieved based on the security design of Mobile IPv6 (see David B. Johnson, Charles E. Perkins, Jari Arkko: Mobility Support in IPv6, RFC 3775). This is secure, but produces long handover delays, and incurs a high signalling overhead.